1. Field of the Invention
This invention relates to solid electrolytic capacitors using an anode made of a valve metal or an alloy thereof.
2. Description of Related Art
Solid electrolytic capacitors are generally produced by laying a dielectric layer, a solid electrolyte layer and a cathode layer one on another in this order on an anode made of a valve metal, such as tantalum, niobium, titanium or aluminum, or an alloy of such a valve metal. Such a solid electrolytic capacitor is used as incorporated into a power supply circuit of a CPU, a peripheral circuit thereof or the like in various mobile terminals, such as mobile personal computers and cellular phones, various image information devices, such as digital cameras, and other electronic devices.
Challenges for these electronic devices are to reduce the ESR (equivalent series resistance) of their solid electrolytic capacitors and to improve the heat resistance thereof, such as heat resistance to reflow soldering. Particularly, surface-mounted chip solid electrolytic capacitors, which are disposed near power supply circuits, conduct large amounts of electric current and therefore are affected by heat generated by peripheral devices and heat of resistance caused by the current flowing through the capacitors. In addition, these chip solid electrolytic capacitors are also required to have heat resistance to reflow soldering process because of elevated reflow temperature (peak temperature: 250° C. to 260° C.) due to use of lead-free solders.
Meanwhile, conductive polymers have been used as solid electrolyte layers in solid electrolytic capacitors. Known conductive polymers having heat resistance include polyaniline, polypyrrole and polythiophene. For example, the techniques disclosed in Published Japanese Patent Applications Nos. H10-321471 and 2008-147392 are known as conventional techniques that use as a solid electrolyte layer a combination of polythiophene having high heat resistance and polypyrrole excellent in rapid polymerization, such as electropolymerization.
In Published Japanese Patent Application No. H10-321471, a first conductive polymer layer is formed of polythiophene or a derivative thereof, and a second conductive polymer layer is formed of polypyrrole or a derivative thereof by electropolymerization. The document discloses that according to the above structure, the capacitance can be retained.
In Published Japanese Patent Application No. 2008-147392, a first conductive polymer layer is made of polythiophene and polypyrrole, and a second conductive polymer layer is made of polypyrrole, wherein the first conductive polymer layer is formed by forming a polythiophene film in islands and then forming polypyrrole to fill in between the islands. The document discloses that the ESR can be reduced because of high-conductivity polythiophene and reduction in contact resistance between the first and second conductive polymer layers due to polypyrroles contained in both the layers.
Published Japanese Patent Application No. 2002-15956 discloses that a precoat layer, a first conductive polymer layer and a second conductive polymer layer are formed on a dielectric layer, and that an example of a specific combination of conductive polymers is a combination precoat and first conductive polymer layers made polypyrrole and a second conductive polymer layer formed by adding carbon fibers to polythiophene. The document discloses that according to the above structure, the stress applied to the capacitor by heat stress due to soldering and the like can be reduced to thereby increase the heat resistance.
However, in the techniques disclosed in Published Japanese Patent Applications Nos. H10-321471 and 2008-147392, the formation of the conductive polymer layer containing polythiophene on the dielectric layer involves poor adhesion of the polythiophene layer to the dielectric layer, resulting in poor heat resistance of the capacitor.
In Published Japanese Patent Application No. 2002-15956, the second conductive polymer layer containing a mixture of polythiophene and carbon fibers is formed on polypyrrole forming the first conductive polymer layer. This structure has a problem in that the heat resistance cannot sufficiently be increased.